This invention relates to a process of preparing a fluorocarbon carboxylic or sulfonic acid, viz., Rf(COOH).sub.m or RfSO.sub.3 H (Rf is a perfluoroalkyl group which may be saturated or unsaturated, and m is 1 or 2), from fluoride of the acid, Rf(COF).sub.m or RfSO.sub.2 F.
Fluorocarbon carboxylic acids Rf(COOH).sub.m having 1 to 10 carbon atoms in the Rf group are industrially important materials since simple derivatives of these acids have various uses. For example, some fluorocarbon carbonyl chlorides Rf(COCl).sub.m are useful as intermediates of medicines and agricultural chemicals or polymerization initiators functioning at low temperatures, and excellent surfactants can be derived from Rf(COOH).sub.m having a relatively large number of carbon atoms. Besides, some fluorocarbon compounds having special uses can easily be obtained by thermal decomposition of salts of Rf(COOH).sub.m. For example, thermal decomposition of C.sub.3 F.sub.7 COOAg gives C.sub.6 F.sub.14, which means dimerization of the perfluoroalkyl group, and thermal decomposition of C.sub.3 F.sub.7 COONH.sub.4 gives a hydrogen-containing fluorocarbon C.sub.3 F.sub.7 H. The obtained fluorocarbons can be used as refrigerant or heat medium.
As to the preparation of Rf(COOH).sub.m, electrolytic fluorination is a generic method which does not need to be modified in fundamentals of apparatus and procedure according to the number of carbon atoms in the Rf group. JP No. 31-268 (1956) shows electrolytic fluorination of carbonyl chlorides or fluorides, R(COF).sub.m or R(COCl).sub.m (R is unsubstituted alkyl group corresponding to Rf) in anhydrous hydrogen fluoride. The reaction product, Rf(COF).sub.m, is recovered as a gas mixed with by-produced hydrogen or, when the product is high in boiling point, as a liquid which is separated from hydrogen fluoride and extracted from the bottom of the electrolytic cell. In general Rf(COF).sub.m are readily soluble in water and rapidly undergo hydrolysis to form corresponding fluorocarbon carboxylic acids Rf(COOH).sub.m. In conventional processes a water scrubber is used to absorb Rf(COF).sub.m in water and hydrolyze the absorbed fluoride, and the resultant aqueous solution is subjected to distillation for isolating the aimed Rf(COOH).sub.m.
In industrial practice of the above process, inconvenience is offered by the coexistence of a considerable quantity of HF with Rf(COF).sub.m recovered from the electrolytic cell. It is impossible to drastically decrease the coexisting HF by merely devising equipment such as low temperature condenser and decanter. It is conceivable to remove the coexisting HF by passing the mixed gas through a tower packed with NaF, but this method is not suitable for industrial application because of high cost and problems about choking of the tower and regeneration of NaF. Furthermore, HF is formed by the hydrolyzing reaction of the carbonyl fluoride as represented by the equation (1), so that existence of HF in the obtained acid solution is inevitable. EQU Rf(COF).sub.m +H.sub.2 O.fwdarw.Rf(COOH).sub.m +HF (1)
Therefore, ordinary metal or glass materials are impracticable for the hydrolyzing and distillating apparatus, and it is necessary to use a very costly apparatus material such as a fluororesin lined material. Besides, complete removal of the coexisting HF is difficult even by distillation so that a problem arises as to purity of the obtained fluorocarbon carboxylic acid.
In preparing Rf(COOH).sub.m having 1 to 5 carbon atoms in the Rf group by the above process, it is a matter of inconvenience for the final distillation operation that the boiling points of the aimed compounds are in the range of from 70.degree. to 160.degree. C. (e.g., CF.sub.3 COOH 71.degree. C., n-C.sub.3 F.sub.7 COOH 119.degree. C., C.sub.5 F.sub.11 COOH 156.degree. C.) and are not very far from the boiling point of water. For this reason it is necessary to remove a large quantity of water by using a very large-scale distillation tower and by consuming a large amount of energy.
Kogyo Kagaku Zasshi (a Japanese journal), 64, 1397 (1961) shows a process having the steps of forming a perfluoroalkylcarbonyl fluoride by electrolytic fluorination of an alcohol in anhydrous hydrogen fluoride, forcing the carbonyl fluoride to be absorbed in water followed by addition of silica and sodium carbonate, and separating sodium salt of the aimed fluorocarbon carboxylic acid by extraction with alcohol. In practice this process will not be economical because of including complicated operations for the solvent extraction and also because of requiring an isolation procedure such as distillation subsequent to the solvent extraction.
Fluorocarbon sulfonic acids RfSO.sub.3 H having 1 to 3 carbon atoms in the Rf group are useful as catalysts for various reactions including Friedel-Crafts reactions, nitration reactions and polymerization reactions.
According to JP No. 30-4218 (1955), a fluorocarbon sulfonic acid of the above general formula is prepared from a corresponding sulfonyl fluoride. The fluoride, RfSO.sub.2 F, is obtained by electrolytic fluorination of a hydrocarbon sulfonyl chloride RSO.sub.2 Cl (R is an unsubstituted alkyl group). The fluoride is gaseous at room temperature and, as a product of electrolytic fluorination, is diluted with a large quantity of hydrogen gas. Therefore the fluoride is first condensed by cooling to a sufficiently low temperature such as -180.degree. C., and the condensate is subjected to a hydrolyzing reaction with a KOH solution under pressure in an autoclave type reactor. This reaction is for converting the fluoride into potassium salt, RfSO.sub.3 K, and the desired RfSO.sub.3 H is obtained by reacting RfSO.sub.3 K with an excess quantity of nearly 100% sulfuric acid and distilling the reaction product.
However, in this process the condensation of RfSO.sub.2 F is disadvantageous to industrial practice because very intense cooling has to be made at large expense of refrigerant or electric power for refrigeration and also because the operation is necessarily conducted batch-wise. Besides, the hydrolysis of RfSO.sub.2 F under pressure in an autoclave or the like entails high costs of equipment and operation. Because of including these inconvenient operations it is very difficult to conduct the overall process in a continuous manner, and inevitably the products become very costly.